Endotoxin was measured by the Limulus assay in plasma and ascites in 46 patients with cirrhosis having demonstrable esophageal varices, of whom 29 had ascites and 17 did not. It was positive in ascitic fluid in 23 (79.3%) of the former group. In plasma, a positive test was obtained in 22 (75.9%) in the group with ascites and only 4 (23.5%) without ascites, the difference being significant (P less than 0.01). Of the 23 positive ascites specimens, 17 showed high titers (greater than or equal to 10(-3) microgram per ml). Hepatic uptake of 198Au colloid was markedly reduced in 11 of the 17 patients with endotoxemia who were studied by scanning. Death occurred within 6 months in 47.8% of the patients with a positive endotoxin test, whereas only 16.7% of those with a negative test died in the same period (P less than 0.05). No hypotension was noted in patients with toxemia and only 2 ran a fever above 37.5 degrees C. Development of tolerance to endotoxin is suspected. A follow-up study has demonstrated sustained endotoxemia in some of these patients.